Ferrites of K and Ba close in composition to the known synthetic compounds KFe22+Fe153+O25 and BaMg2Fe163+O27 were found in a thin vein, filled with magnesioferrite and khesinite, in pyrometamorphic flamite–gehlenite hornfels of the Hatrurim Complex, Palestinian Autonomy. Both ferrites are characterized by a modular structure composed of 5-layered spinel (S) blocks interstratified with R-blocks. The R-modules of K-ferrite are of the β-alumina type whereas those in Ba-ferrite are of the magnetoplumbite type. Rare grains of K-Ba-ferrite of intermediate composition with the empirical crystal chemical formula (EPMA): (K0.57Ba0.38Na0.05)∑1(Fe14.083+Mg1.42Al0.80Zn0.59Ni0.16Ca0.14Cu0.09Mn0.03Ti0.03Si0.02)∑17.36O25.54⁠, were also discovered. The structure of the natural mixed potassium-barium ferrite was investigated by single-crystal X-ray diffraction. The hexagonal space-group symmetry (⁠P6‾m2⁠, Z = 2) with a = 5.9137(2) and c = 33.1450(15) Å is different to that (P63/mmc) of the end-members and yields alternate stacking of β-alumina type KMg2Fe15O25 and magnetoplumbite type BaMg2Fe16O27 “supermodules”, each extending ½ of the unit cell along c. Raman spectroscopic study confirms the mixed character of K-Ba-ferrite structure. K-Ba-ferrite formed at non-equilibrium conditions and may be interpreted as an example of a nano-dissipative structure.

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